The screenplay for the film was originally completed by Sandor Stern, though it was so significantly rewritten by star Cassavetes during production that a pseudonymous "George Franklin" received the official credit as screenwriter.

Upon its theatrical release in 1982, the film received largely unfavorable reviews,[7] with many citing its plot and overt violence as weaknesses.

In a lake at a rock quarry in rural Wisconsin, a young woman, Mandy Pullman, and her boyfriend, Roy, are swimming.

The two spend the night at the lake camping, but are attacked by an unseen figure; Roy is killed, and Mandy violently raped.

While he is there, a young woman is raped and murdered in the downstairs bathroom of the theater, and the metal stall door is found nearly bent in half.

She confides in Sam that she discovered historical records detailing Satanism and similar crimes occurring throughout the town's history.

As Laura takes Jenny upstairs to go to bed, Sam attempts to induce Tim's dream to prove its connection to the murders.

Tim goes into a seizure-like state and runs upstairs into Jenny's room where he tries to attack Laura with a dagger given to him by Agatha, but Sam intervenes and stabs him to death.

As Laura embraces Sam, he looks over her shoulder to see Jenny's dead body lying on her bed, blood pouring out from between her legs.

"[24] Ed Blank of the Pittsburgh Press criticized the film's plot and direction, but conceded the musical score as "chilling.

"[25] Rick Kogan of The Journal News deemed the film "mindlessly bloody, crudely made and distasteful," concluding that it was "vile and mean-spirited.

"[26] The Atlanta Constitution's Eleanor Ringel similarly noted the film's overt violence, deeming it "disturbingly nasty, [but] not so poorly done that you can completely shrug it off."

"[27] TV Guide published an unfavorable review of the film, writing: "Though potentially interesting, The Incubus suffers from a moronic script, which tosses the audience a dozen different diversions in the name of suspense.